Electron discharge tube system



May 6, 1941. J. vms

ELECTRON DISCHARGE TUBE SYSTEM Filed March 18, 1939 4 Q A M 0% 1 m a m P w L: W I y r r Eacrl.

I l I I l l l l l l I INVENTOR M W XATTA I I l I I I Patented May 6, 1941 UNITED STATES PATENT OFFICE 2,240,635 ELECTRON DISCHARGE TUBE SYSTEM Jack Avins, New Dorp, N. Y. Application March 18, 1939, Serial N0. 262,601

4 Claims.

This invention relates to electron-discharge tube systems and more especially to systems having the characteristics of a balanced amplifier.

A principal object of the invention is to provide an improved amplifier system employing a novel method of phase inversion.

Another principal object is to provide an improved balanced amplifier.

Another object is to provide a multi-stage balanced amplifier employing a novel input circuit for one or more of the stages.

A feature of the invention relates to a phase inversion circuit employing grid-controlled electron tubes which have a common cathode return circuit of high impedance. As a result 01 this feature, the circuit is capable of eflecting phase inversion and at the same time maintaining stabilized operation, both of which functions are accomplished without the great reduction in gain which ordinarily occurs when a high impedance is inserted in the cathode return circuit of an electron-discharge tube amplifier.

Another feature relates to a balanced amplifier circuit employing a plurality of grid-controlled vacuum tubes having a common cathode return circuit of high impedance whereby the operation of the amplifier is stabilized and the necessity of careful control of the circuit parameters, tube characteristics, supply voltages, and the like, is obviated. This feature is of special advantage where one or morestages oi the amplifier are direct" coupled to each other. Accordingly, an ancillary feature relates to an improved direct coupled amplifier.

A further feature relates to the novel organization, arrangement and interconnection of parts which constitute an improved and stable amplifier system having a balanced output in at least one stage which stage is excited by an unbalanced input signal.

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims. The invention in certain of its phases is in the nature 01. an improvement on the type of system disclosed in page 79 of Electron Tubes in Industry by Keith Henney, a book publishedby McGraw-Hill Book Company, Inc., New York.

In the drawing which represents certain preferred embodiments of the invention- Fig. 1 is a schematic diagram of an amplifier or phase inverter according to the invention.

Fig. 2 is an equivalent circuit diagram explanatory ofthe operation of Fig. 1.

a battery 8. The plate or output anode 9 of tube I and the corresponding anode I 0 of tube 2 are connected through the series resistors ll, l2, the electrical midpoint or other suitable point of which is connected to the positive terminal of source 8. A common ground connection is made to a suitable point on source 8.

The signals to be amplified or measured are impressed across the input terminals l3, l4, and the input resistance or impedance I5. Impedance I5 has one end directly connected to the controlgrid l6 of tube l and the other end is connected to the common ground or base potential for the system. The control-grid ll of tube 2 is preferably directly connected to this common ground.

Any form of load device schematically represented by the block 20 is H, l2.

To explain the operation of the circuit in Fig. l, the result of applying a voltage to the input will be analyzed and an expression developed for the change in the cathode current of tubes I and 2.

that when the value or the common cathode resistance 1 is properly related to the other circuit parameters, then the change in the plate current of tube 2 will be approximately equal to the change in the plate current of tube I and opposite in sign. The circuit thus provides a balanced amplification of the input voltage; the

AE=incremental voltage applied to the grid of tube I,

AE1=change in the potential difference between the grid and cathode of the tube I resulting from the applied AE,

AEa=change in the potential difference between the grid and cathode of tub 2 resulting from the applied AE,

R1: cathode resistors of tubes l and 2 used to obtain degeneration where the latter is considered desirable, I

R2: common cathode resistor of tubes I and 2,

Rs: plate load resistors,

AI1=cliange in the plate current or tube I resulting from the applied AE,

AIz=change in the plate current of tube 2 resulting from th applied AE,

p=arnpliflcatin factor of tube I; also of tube 2,

re plate resistance of tube I; also or tube 2.

R4= defined for convenience as the sum of Rp and R3.

Eliminating AEi from (3) and AE:: from (4), and rearranging terms,

From (6) All Raw From (5) and (6) Some general conclusions can be drawn from theabove equations. From (7 AI. ab d- Consider the case where no degeneration is used, that is R==0. Then QL R=O+ 4 (13) AI. R.o+

From the viewpoint of balanced output it is desirable that AI: should be equal in magnitude to A11. To obtain this condition, it follows that or Ra(p.+1) must be large in comparison with R4. The magnitude oi A11 is of importance for this condition.

(15) For convenience let Rahal-l) ==KR4 From (10) For the case which is of interest at the moment, where K 1 pAE(1+%) AI (2+K)R| pAE (17) AI, TR

Since MAE R4 represents the maximum change in plate current which can be obtained, it follows that the change in plate current of V1, A11, is one-half the maximum obtainable value. But for the condition under discussion, AI==AI1, so that the total amplification obtainable from the plate 0! tube 1 to the plate of tube 2, is equal to the am plification which would be obtained by a single tube.

The relations developed above have been investigated experimentally and found to check.

Referring to Fig. 1, it will be noted that the phase inversion described mathematically above is obtained with a small number of circuit components and with symmetrical design. The comparatively high value of cathode impedance used not only efiects the phase inversion but also makes for a high degree or stability since a sellregulating action takes place which tends to minimize the eflfects of changes in supply voltages and circuit parameters.

Related to the high degree or stability characteristic o! the circuit is the reduction of grid current which accompanies the use of a high value 01 cathode resistance. This is important in the design of high input resistance electronic devices, where the circuit being described can be used to great advantage.

This circuit is not to be confused with circuits which upon superficial examination appear to be similar. In circuits of the prior art, the sec- 0nd tube is essentially an inactive or dummy tube which is used to balance out the steady plate current and is referred to as such in the literature. In the present circuit however, a large impedance is intentionally inserted in the grid-cathode circuit common to both tubes so as to make the previously inactive tube active, with the attendant advantages described previously.

As is shown in Fig. 3, the circuit of Fig. 1 can be used as a phase-inverter to couple an input circuit capacitively to a suitable amplifier 0! known construction. The part of this figure within the dotted rectangle is the same as that of Fig. 1 and the corresponding parts of the two figures bear the same designations. Since this portion of the amplifier can be designed to have a balanced output, it can be coupled through suitable coupling capacitances 2|, 22, to any well-known form of amplifier represented by block 23. In this application the advantages of the circuit are its high stability, balanced operation, and the low percentage of hum and distortion in the output.

The principle of returning the cathodes of two tubes through a high impedance or resistance and compensating for the correspondingly high negative bias by returning the control grids to a suitably high positive point in the power supply can be used to advantage in the design of a modified push-pull amplifier. Such an amplifier is schematically illustrated in Fig. 4, it being assumed that the control grids 24, 25, of the tubes 26, 21, receive a. balanced signal excitation rep resented diagrammatically in the drawing by the symbols +AE and 'AE. The cathodes 30, 3|, are connected together by resistors 32, 33, the junction of which is connected through the high resistance 34 to the negative end of the power supply 35. The grids 24, 25, are returned to a point of the supply 35 which point may be grounded and is positive with respect to the return point of the cathodes, the resultant potential on the grids being the proper negative value for the desired operating condition.

For the condition of balanced excitation which is being described, the above Equation 5 remains the same, and Equation 6 becomes From (19) and (20) (R1+ z) (n-i- +R4 1+ 2)(#+ 4P (I++ Considering the case where no degeneration is used, that is,

4 and it will be noted that A11 and AI: are independent of the value of resistance 34. In other words, there is no reduction of gain as a result of using a high value of resistance in the oathode circuit. However, as in the circuit of Fig. 1, the use of this high value of common cathode resistance is desirable in that it makes possible a high order of stability.

By combining the phase inverter arrangement of Fig. 1 with one or more stages of push-pull amplification of the type shown in Fig. 4, it is possible to provide a highly stable direct-coupled amplifier which does not require a critical control of the circuit parameters and supply voltages which has heretofore been one of the well-known limitations of conventional direct-coupled amplifiers. In this application the circuit being disclosed makes possible the design of a symmetrical balanced amplifier in which the output electrodes are connected directly to the grids of the following stage. Although the circuit pa rameters can be chosen so that an input voltage will cause voltage changes 01' sensibly equal value but opposite polarity at the two plates of the first stage, deviation from this operating condition can be compensated for by tapping one or the load impedances or by other suitable means. Such an amplifier is shown schematically in Fig. 5, wherein the part enclosed within the dotted rectangle 39, is the same as the part within the dotted rectangle of Fig. 3; while the part within the dotted rectangle 40, is the same as Fig. 4. In Fig. 5, the cathodes 4|, 42, are connected together through the resistors 43, 44, and the common junction is connected through the resistor 45 to the negative terminal of. the power source 46. The input signal is applied to the control grid 41 of tube 48 and the grid 41 is returned to a base potential point indicated by the ground connection; the control grid 49 oi! tube 50 is returned directly to this same point. By reason of the phase-inverting action of the tubes 48, 50, and associated circuit, the output circuit is excited in balanced relation, this output circuit including the resistors 5|, 52. The balanced out put is applied directly to the control-grids 53, 54, of the tubes 55, 55. The cathodes 51, 58, are connected together through the resistors 59, 50, and the common cathode connection is connected through the high resistance 5| to a suitable point in the common power supply 45 which point is negative with respect to the steady potential of the grids 53, 54. It will be noted that all voltages can be obtained from a single power supply and that the amplifier is more stable than the conventional asymmetrical directcoupled amplifier because of the self-regulating action and because feedback through the power supply is substantially eliminated. Any suitable load device 52 can be energized by the output of the amplifier.

While certain specific circuits and apparatus have been described, it will be understood that the invention is not limited thereto and that various changes and modifications may be made without departing from the spirit and scope of the invention. For example, while Fig. 5 shows only two stages, a greater number of stages may be employed. Furthermore, while the various embodiments disclose a high impedance in the form of a resistance connecting the cathodes to a common point in the potential source, it will be understood that this high impedance may take the form of an inductance or a combination of inductance, capacitance, and resistance to provide the necessary high impedance.

What I claim is:

1. A phase inverting arrangement comprising an input circuit, an output circuit, a pair of electron discharge tubes linking said circuits, said tubes having a cathode, grid and anode electrodes, a source of steady operating potential for the electrodes of said tubes, a load impedance connecting the anodes of said tubes, impedance means connected between the cathodes of said tubes, and means to invert the phase of one anode with respect to the other anode in response to a signal impressed upon the grid of one tube, the last-mentioned means including a high impedance connecting a. median point on said impedance means to a point in the source or operating potential which point is negative to the point of return of said grids.

2. In an electron tube system, an input'circuit unbalanced with respect to ground, an output circuit balanced with respect to ground, and ampliner means linking said circuits, said amplifier means including a pair of discharge tubes having grid, cathode and anode electrodes, the grid of one of said tubes being connected to said input circuit and the grid of the other or said tubes being grounded, a resistor connected between said cathode electrodes, common impedance means in circuit with said cathode electrodes and connected to a median point on said resistor for app yin a voltage to the cathode of said other tube which is in phase opposition to the voltage applied to said input circuit, said anode electrodes being symmetrically coupled to said balanced output circuit.

3. A phase inverting amplifier comprising a grounded and an ungrounded input terminal, a pair of electron discharge tubes having cathode, grid and anode electrodes, means coupling the grid electrode oi one of said tubes to said ungrounded input terminal, means grounding the grid electrode 0! the other of said tubes, a resistor connected between said cathode electrodes,

common impedance means connected between a median point on said resistor and ground, said connection including a biasing battery, a source of positive potential for said anode electrodes, and a pair of load impedances connected between said source and said anode electrodes.

4. A phase inverting system comprising an input circuit, an output circuit, a pair of electron discharge tubes having cathode, grid and anode electrodes linking said circuits, said output circuit including a load impedance connected between the anodes oi said tubes, a symmetrical connection between said cathode electrodes and said load impedance, said symmetrical connection including impedance means connected between said cathodes, a median point on said impedance means being connected to said load impedance through a common high impedance and a. source of high potential for said anodes, and connections for returning the grid electrodes of said tubes to a point in said source which is intermediate the potential of the terminals thereof.

JACK AVINS. 

